The specialized cytoarchitecture of the hematopoietic microenvironment is created by discrete cell-cell and cell-matrix adhesive interactions that are tightly regulated by lineage-specific expression of adhesion molecules. The earliest human hematopoietic progenitor cells (HPCs) are characterized by the absence of lineage-specific markers and expression of the cell surface molecule, CD34. A variety of adhesion molecules are expressed on HPCs, including CD44 (the “hyaluronic acid receptor”, also known as H-CAM), members of the integrin (e.g., LFA-1, VLA-4) and immunoglobulin (e.g., ICAM1) superfamilies, and a member of the selectin family, L-selectin. L-selectin (CD62L) is a calcium-dependent, carbohydrate binding protein in a family of adhesion molecules that also includes E-selectin (CD62E), expressed on activated vascular endothelium, and P-selectin (CD62P), found on both activated platelets and endothelial cells. Selectin-mediated interactions are critical not only for the rapid and efficient recruitment of leukocytes at a site of injury, but for steady state, tissue-specific homing as illustrated in: (1) lymphocyte homing to peripheral lymph nodes, (2) cutaneous tropism of human skin-homing T-cells and (3) hematopoietic progenitor cell (HPC) entry into bone marrow.